1. Field of the Invention
The present invention relates to a communication apparatus that switches, by roaming, between access points that relay wireless communications.
2. Description of the Related Art
IEEE 802.11, which is a set of standards for a local area network (LAN), defines a mutual wireless communication system between a base station (access point (AP)) and a terminal station (station (STA)). The AP is a parent station having a physical management range. The AP manages a plurality of STAs connected to the AP, and thereby controls communications between the AP and the STAs.
Because a range of a radio wave emitted from the AP is confined, to not loose an STA, a plurality of APs are installed, in general, in such a manner their ranges of the radio waves (hereinafter, “coverage areas”) overlap. With this configuration, a mobile personal computer (PC) that is transferring data, moving from a coverage area of an AP 1 to a coverage area of an AP 2 can maintain an on-line state by roaming, i.e., switching from the AP 1 to the AP 2.
Each of the APs is allocated with identification data called “service set identifier (SSID)”. The SSID is used by an STA to identify a wireless communications network to which it can be connected. A group of APs can be allocated with the same SSID. If, for example, a network on the third floor of a certain building has an SSID “Floor3”, then the SSID “Floor3” is allocated to all of the APs installed on the third floor.
If the SSIDs are decided on a floor-to-floor basis as in the above example, as long as the STA moves within the floor, the on-line state is maintained by the roaming operation without problems. However, the roaming operation cannot be performed when the STA moves to another floor because the SSIDs are different for the two floors. Therefore, when the STA moves to another floor, the on-line state is disconnected. To reestablish the on-line state, the STA performs a re-connection process by identifying the SSID of the floor to which the STA has moved. This situation is undesirable because data cannot be transferred during the re-connection process.
To avoid the above-described undesirable situation, one approach is to allocate a single SSID to all the APs in a building. Usage of the single SSID makes it possible to maintain the on-line state even when an STA moves from one floor to another.
A typical method of the roaming is described below. To periodically scan for connectable APs, an STA transmits a request message (hereinafter, “probe request frame”) during the data transmission. Upon receiving the probe request frame, each of the connectable APs returns a response message (hereinafter, “probe response frame”) to the STA. The STA decides, based on the probe response frame, one of the APs as a target AP to which the STA switches by the roaming (hereinafter, “next AP”).
If two or more connectable APs exist simultaneously, in a technology disclosed in Japanese Patent Application Laid-open No. 2001-352569, the connectable AP that returns the probe response frame with the strongest radio wave is selected as the next AP. More particularly, the STA determines whether the SSID extracted from the received probe response frames is the same. If the SSIDs are the same, the STA connects itself to an AP having the stronger radio wave. For example, if the STA determines, while it is currently connected to the AP 1, that the radio wave received from the AP 2 is stronger than the radio wave received from the AP 1, the STA switches from the AP 1 to the AP 2. On the other hand, if the STA determines that the radio wave received from the AP 1 is stronger than the radio wave received from the AP 2, the STA maintains the connection with the AP 1.
However, selection of the next AP based on the intensity of the radio wave can cause frequent switches, which may disadvantageously result in a loss of the data transmission time.
Assume, for example, that the single SSID is used on every floor of the building, and the STA is installed by a window on the third floor. The STA may receive the radio wave coming from another floor. In general, the intensity of such a radio wave coming through outside of the building is unstable because of various external factors such as multipath, reflection, and phasing. If the STA switches between the APs in comparison with the intensities of the radio waves only, the STA may frequently switch between the AP on the third floor and the AP on another floor. Although shorter than the stop period required by the re-connection process, the roaming operation stops the data transmission for a while.